Corona discharge device

ABSTRACT

A corona discharge device for use in the electrophotographic field having an element which is in the form of a tape, has at least one conductive layer at its discharge edges, is so movable in the direction of its discharge edges that the corona discharge element can be moved at least by the length of the discharge zone and in that at least one cleaning device is provided for the corona discharge element, preferably outside the discharge zone. The tape is an endless belt having at least one edge which can discharge with that edge exhibiting various profiles. The belt also may be shaped with a depression or carry a guiding element to prevent lateral movement of the tape.

United States Patent 1191 Marx et a1.

[ CORONA DISCHARGE DEVICE [75] Inventors: Gerhard Marx, Hahn/Taunus;

Hermann Frank, Sulzbach, both of Germany [73] Assignee: HoechstAktiengesellschatt,

Wiesbaden-Biebrich, Germany 22 Filed: Nov. 14, 1974 211 Appl. No.:523,751

Related US. Application Data [63] Continuation of Ser. No. 372,305, June21,1973,

abandoned.

[30] Foreign Application Priority Data June 22, 1972 Germany 2230483[52] US. Cl. 250/324; 250/325; 317/262 A [51] Int. Cl G03g 15/00 [58]Field of Search 250/324, 325, 326; 317/262 A [56] References CitedUNITED STATES PATENTS 2,856,533 10/1958 Rosenthal 250/325 3,169,8862/1965 Simm 250/325 3,499,143 3/1970 Martin.. 250/324 3,546,545 12/1970Sato 250/324 June 3,1975

OTHER PUBLICATIONS Corona Unit Cleaning Device, Voit, IBM TechnicalDisclosure Bulletin, Vol. 11, No. 8, January 1969, pp, 1025 and 250-324.

Primary Examiner-Craig E. Church Attorney, Agent, or FirmStevens, Davis,Miller & Mosher 5 7] ABSTRACT A corona discharge device for use in theelectrophotographic field having an element which is in the form of atape, has at least one conductive layer at its discharge edges, is somovable in the direction of its discharge edges that the coronadischarge element can 25 Claims, 33 Drawing Figures SHEET PATENTEDJUH3|9rs sum PATENTEIJJUH 3 I975 F i g 3 F ig.3b 1 31a FATENTEI] JUH 3 msSHEET Fig.5

SUB-1200M 10B HIUUV SUB-1500M vv no no on 21 1 on on 5 1 CORONADISCHARGE DEVICE This is a continuation, of application Ser. No.372,305, filed June 21, 1973 now abandoned.

The present invention relates to a corona discharge device having atleast one corona discharge element.

Corona discharge devices are required particularly in theelectrophotographic field. With these corona discharge devices,insulating surfaces are charged or their charge is reversed. In an oftenemployed process the insulating surfaces are photoconductive layerswhich are charged and, after each uniform charge, imagewise exposed tolight and the remaining charge image is made visible by means of atoner.

Particularly in those cases in which a negative charge of thephotoconductive material is necessary, there rises the problem that theknown corona discharge devices lead to an undesirable non-uniformcharge. Particularly serious are those non-uniformities when theelectrophotographic copying device permits development of continuoustones, i.e., when not only the presence of a charge difference plays apart but also the extent of the charge difference leads to differingblackening in the copy. In reverse processes, in which discharged areascorrespond to black areas of the copy, charge faults lead toparticularly undesirable tinted lines or spots whereas such faults inthe normal pro cesses lead to image blanks.

Numerous attempts have been made to solve those problems. According toone attempt, a known wire corona is used and the wire in the corona iscirculated or rotated about its own longitudinal axis. Such devices aredescribed in US. Pat. No. 2,856,533, for example. Another solution isdescribed in this publication, in which the discharge wire is caused toexecute an oscillating movement. Another device is described in GermanAuslegeschrift No. 1,128,293. It consists of an endless belt support onwhich individually arranged discharge pins are positionedperpendicularly to the belt surface. These discharge pins are movabletogether with the belt support transversely to the direction of movementof the surface to be charged.

Another not published solution has been suggested, in which a rotatingroller of small diameter is used. In this case, the electrode consistsof a roughened cylindrical body. With this device, the problemencountered with the first described corona discharge device, viz. ofburning-off and of the undesirable non-uniformity of discharge, has beensolved in that the rotating cylindrical electrode is cleaned on the backside, i.e., on the side away from the surface to be charged.

The last described corona discharge device has led to satisfactoryresults in the case of small widths of surfaces to be charged but, forcharging wider surfaces, particularly the trueness of rotation of such athin roller could not be achieved with reasonable technical means.

The object of the present invention thus is to provide a coronadischarge device with which simultaneously two problems are solved in asatisfactory manner, i.e., to provide an apparatus with which 1.surfaces of large widths can be charged in an extremely homogeneousmanner, and

2. the difficulties connected with contamination of the dischargeelement by dust, cutting residues, burning-off or ZnO particles areovercome; particularly in the case of negative charges and in thereverse process.

The present invention provides a corona discharge of the initiallydefined kind, which is characterized in that the corona dischargeelement is in the form of a tape, has at least one conductive layer atits discharge edges, is so movable in the direction of its dischargeedges that the corona discharge element can be moved at least by thelength of the discharge zone and in that at least one cleaning device isprovided for the corona discharge element, preferably outside thedischarge zone.

The use of tape-shaped material as the discharge element of such acorona has the double advantage that cleaning of the tape fromburning-off residues is possible in a simple manner and thatsimultaneously wide surfaces can be charged. The latter results from thefact that such tape material may be secured against any kind ofdistortion and vibration by tightening faster, by shaping or by supportsor bearings.

In accordance with the present invention, it is preferable to use anendless belt as the tape-shaped corona discharge element and tocirculate it about two rotatably mounted rolls'or corresponding guides.But it is also possible to work with a larger tape supply and to movethe same to and fro between two coils. The switching mechanism in anautomatically operating apparatus may be similar to that known withtypewriter ribbons.

it is is preferable to shape the tape at least in the discharge area inthe longitudinal direction. This may be performed in that a shape isimpressed into the tape. But is also possible, before or after or evenalong the actual discharge path, to draw the tape through appropriateprofile guides which give the tape the corresponding shape.

The tape-shaped discharge element also may be an only partially elastictape or a tape provided with links.

To the tape there may be applied special discharge layers. Furthermore,there may be applied conductive layers which are insulated from thedischarge layers and serve for discharge control, for example.

When the tape-shaped discharge element is a tape capable of dischargingat both edges it is possible and preferable in accordance with theinvention to move the tape between the two longitudinal edges, e.g., inthe center between guides over the entire discharge area on the backside and possibly also on the front side. When the tape is provided witha longitudinal corrugation', for example, which is held by correspondingshaped parts of the guides every movement of the discharge element isimpossible except that in the longitudinal direction of the tape.-

The tape-shaped discharge element may be tightened and guidedparticularly easily when it is a very thin tape which, for the purposeof providing mechanical strength, has a wire or sheet-metal stripbetween the edges and firmly connected with the tape. When the thickertape material is used, the discharging tape edges are sharp-edged orpointed.

In order to achieve increased flexibility of the tape, the wire orsheet-metal strip may be disconnected at certain distances. Thedischargeable tape material then consists of preferably 0.2 to 15 mmlong elements abutting on one another. The joints between the elementsmay be similar to locks to be able to transfer longitudinal tensions atleast in the stretched state.

In accordance with the present invention, a corona discharge device ispreferred in which an auxiliary electrode is coordinated to thecirculating tape-shaped discharge element. This auxiliary electrode mayconsist of a row of needles, the needle points having a constantdistance from the tape edge. The movable tape-shaped discharge elementmay be such that it discharges on two sides. In this case, it is adouble corona with one discharge element. This increases the uniformityof charge. If desired, the tape may be toothed or corrugated on theedges.

Finally, it is also possible to have the auxiliary electrode in the formof a tape and to circulate it. In this case it is advantageous when theauxiliary electrode or electrodes can be circulated in a directionopposite to the direction of rotation of the discharge element. But itis also possible to circulate these at least two electrodes at differentspeeds.

For cleaning the discharge element a doctor blade is preferred. It maybe of metal or also of a softer material, such as velvet leather orbristle-like material. It may also be a floating knife.

With the corona discharge device of the present invention it is easilypossible to suck off any undesirable ozone.

Further details of the present invention can be seen from the followingdescription of exemplary embodiments by way of the figures of thedrawings in which:

FIG. 1 is a diagrammatic sectional view of a corona according to thepresent invention;

FIG. 2 is a sectional view on the line II-II through the apparatus shownin FIG. 1;

FIGS. 3a-3m show several different shapes of the tape-shaped dischargeelement;

FIG. 4 is a sectional view of another embodiment of a corona accordingto the present invention;

FIG. 5 shows a specific embodiment of a corona with deflectingelectrodes;

FIGS. 6a-6k show plan views of examples of discharge elements;

FIGS. 7a-7dshow side elevations of examples of articulated dischargeelements;

FIG. 8 is a perspective partial view of another corona discharge device;and

FIG. 9 is a sectional view of another embodiment of a corona dischargedevice according to the present invention.

FIGS. 1 and 2 are two sectional views of a corona discharge'device inwhich the corona discharge element 1 is an endless circulating belt. Thecorona discharge element 1 consists of a metal tape of a thickness of afew ,um and a width of some mm. A flexible plastics tape provided withan electroconductive layer, e.g., vapor-deposited tungsten, also may beused.

The metal tape is provided with a corrugation 3 the depth of whichcorresponds at least to the tape thickness. The tape 1 is passed aroundtwo drivable rolls 4 and 5 andruns on the back side of the actualdischarge path through cleaning material 6. As indicated in FIGS.

10 of the circulating tape 1 are each faced by a row of densely arrangedneedles II and 12, respectively.

The plastics block 13 is the actual carrying element of the coronadischarge device. The two needle rows 1] and 12 are secured to thisplastics block by means of two angle sheets 14 and 15. The guide rolls 4and 5 are also positioned in the block 13. On its reverse side, theplastics block is provided with a recess 16 in which is arranged thecleaning material 6 and in which runs the discharge element 1. Thiscleaning channel 16 of the block 13 is closed with a lid 17.

On its front side, the block 13 has two channels 18 and 19 between whichis a bridge 20. This bridge 20 is narrower than the width of thedischarge element 1. Furthermore, the bridge 20 has a longitudinalrecess the form and depth of which substantially correspond to thecorrugation 3. The tape 1 is on this bridge 20 and is secured by thecorresponding recess against lateral displacement. A plastics plate 21which, if desired, may additionally have a rib engaging the corrugation3 is connected on both ends with the plastics block 13. This plate 21serves to secure the tape against displacement transverse to thedirection of movement also in those cases in which the corona isoperated when the material to be charged or discharged is beneath thecorona.

For removing possibly disturbing ozone, one or more longitudinalchannels 23 may be provided in the plastics block 13, which, via severalbores 24, are in connection with the recesses 18 and 19. Through thebores 24 and the channels 23, the ozone may then be removed by means ofa suction device.

By the operation of such a corona discharge device it has been foundthat it is possible to solve the burningoff problems by the cleaning ofthe tape-shaped material as well as to overcome those difficulties whichoccur when a corona for larger widths of the material to be chargedshall be designed. The tape-shaped discharge element offers theconsiderable advantage that it may be considerably tightened and alsopositioned between the discharge edges.

Some examples of possible cross-sections of the tapeshaped dischargeelement are illustrated in FIG. 3. The direction of movement of thetape-shaped discharge element is normally perpendicular to the drawingplane.

In the simplest case as illustrated in FIG. 3 in the first line, rightcolumn, the tape-shaped discharge element 1c has a rectangularcross-section the width considerably exceeding the thickness. Thethickness of the tapeshaped discharge element may be 0.02 mm, forexample, whereas the width is 4 mm. In order to further increase thetensile strength of the discharge element and to guarantee theuniformity of running of the discharge element, the latter may beprovided with reinforcing elements. The first line of the profiles ofFIG. 3, i.e., FIGS. 3a and 3b, shows two examples of a discharge element1a and 1b reinforced with a wire 31 and 31a, respectively. In the firstcase, the wire 31 is on the side facing the material to be charged ofthe discharge element 1a, whereas in the second case 31a is fastened onthe side of the discharge element away from the material to be charged.This applies similarly to the second and third embodiments of thecross-sections shown in column 1 of FIG. 3, i.e., FIGS. 3d and 3g. Areinforcing metal sheet strip 32 and 320 are used instead of a wire 31and 31a. The other variations shown in line 3 of FIG. 3, i.e., FIGS. 312and 3j, show discharge elements II: and 1 provided with corrugations 3aand 3. Finally, in the fourth embodiment of column 1 of the variationsshown in FIG. 3, i.e., FIG. 3k, the tape-shaped discharge element lk isprovided with a reinforcing element 33 having a narrow base 34 and ahead 35 of a semi-circular cross-section. A corresponding guidingelement 33a has the discharge element lu illustrated in FIG. 3 1. Inthis case the discharge element In has the form of a blade.

In these variations the guiding and holding element 31 or 31a or 32 or32a or 33 or 33a as well as the corru gations 3 and 3a serve toguarantee the uniformity of circulation of the discharge element 1a, lb,1d, lg. 1k, Ill, 1 or 1/2, transversely to the direction of movement. Inthe ready corona discharge devices, the corresponding elements areengaging complementary profile guides. The guiding elements 31 or 31a or32 or 32a or 33 or 33a increase the tensile strength of the dichargeelement and generally take up a large portion of the tension.

The second and third examples in the second line of the profilevariations of the discharge element 1e and If, i.e., FIGS. 3e and 3fshow curved tape crosssections. In one case, the tape 1e is concave andin the second case convex with respect to the surface to be charged. Thecurve of the discharge element can be achieved by internal tensions inthat the tape is passed through correspondingly shaped guide slits atboth ends of the discharge length. By this curvature of the tape, a verystable position without too high a tension is achieved with respect tothe distance of the discharge element from the surface to be charged aswell as in the direction transverse to the tape movement. Between theslits for the curved profile and the guide rolls, there is provided asufficient distance so that the tape is not exposed to exceedingly highstresses during conversion from the plane into the curved state and viceversa.

The last tape cross-section variation shown in FIG. 3m is a compositedischarge element. The actual discharge element 1c which is connectedwith the voltage source 8 has insulations 25 and 26 on both sides. Onthese insulations, there are conductor surfaces 27 and 28 connected toauxiliary voltage sources 29 and 30. By means of these conductorsurfaces 27 and 28, it is possible to influence the discharge derivingfrom the discharge element 10. With these electrodes it is possible tospread the discharge streams apart or to concentrate them.

FIG. 4 shows the cross-section of another corona discharge device. Thefigure corresponds to FIG. 2. In this case, not only the dischargeelement 1 is in the form of a circulating tape but also the auxiliaryelectrodes 11a and 12a are in the form of such circulating tapes. Allthree tapes are provided with corrugations 3 or 1 l and 120,respectively, which cooperate with complementary recesses or profilesand exclude lateral running inaccuracies. The tapes Ila and 12a run inthe direction opposite to the direction of rotation of the dischargeelement 1 or in the same direction at different speeds. Cleaning devices61 and 62 also are provided for the counter electrode tapes Ila and 12aon the reverse side of the plastics block 13a, which devicessubstantially correspond to the cleaning device 6 for the dischargeelement 1 with respect to design and mode of action. The discharge edgeshave the numerals 9, 10, in FIG. and 71 and 72 in FIG. 4.

FIG. 5 is a sectional view of another embodiment of a corona dischargedevice which substantially differs from that illustrated in FIG. 2 byguide or control electrodes 40-44. The electrodes 40 and 41 are eacharranged on the side of the discharge path away from the surface to becharged. The further control electrodes 42, 43 and 44 are so arrangedalong the discharge path that the charging current can pass betweenthem. It is also possible to use a control grid instead of the controlelectrodes 42-44.

The voltages indicated in FIG. 5 are so selected regarding height andsign that optimum charge with respect to current yield and uniformity isachieved. The voltage is supplied preferably through severalelectronically controlled supply parts.

FIG. 6 shows ten examples of discharge elements 1m to It, 50 and 50a inwhich one or both edges have sawtoothed profiles. It is also possible towork with tapeshaped discharge elements the edges of which, viewed inthe direction of rotation, are slightly corrugated (not shown).

The discharge electrodes shown in FIGS. 7a 7d differ from those hithertodescribed in that they consist of rigid individual elements. Theseindividual elements 50b may consist of thicker sheet metal material andhave discharge edges 51. In the example shown in FIG. 7a, the rigidindividual elements 50b are mounted by means of fastenings 53 (rivets orspot welds) on a flexible tape 52 passed around the guide roll 4b.

In FIG. 7b, the round guide roll is replaced by a hexagon roll 4c theedge length of which corresponds to the length of the individual element50b. In this case, the individual element 50b may be connected atseveral places or over its entire area with the guiding tape 52. Theindividual elements may be needles 50, 50a or 50c (round or edged)provided with flat or pointed ends and mounted densely beside oneanother on the guiding tape 52 (see FIGS. 60 and 611 as well as FIG.7d).

Instead of a guiding tape 52, there may also be elastic or mechanicaljoints 54 between the elements passing around a guide roller 4d. This isshown in FIG. 7c. The joints may be similar to locks. When they arespring locks a special tightening device for the tape is no longernecessary.

The above described corona discharge device provides the possibility tocharge surfaces of very large widths considerably uniformly as well asto solve the problems involved by burning-off in a very satisfactorymanner. This possibility is provided by the tape-shaped material,cleaning as well as uniform vibration-free movement being possible dueto the tape structure.

FIG. 8 shows a corona discharge device in which the circulating endlesscorona discharge element In is arranged perpendicularly to the web 59 tobe charged. The tape In is passed around two rolls 4e and 5e (the latteris not shown in FIG. 8) and driven by one of the rolls 4e and 52. Thetape In is connected to a voltage source 8. A tape-shaped endlessauxiliary electrode 11b is likewise arranged perpendicularly to thesurface of the paper 59 and includes the corona discharge element In.The auxiliary electrode 11b is driven by means of rolls 57 and 58. Inthe discharge zone, both tapes 1n and 11b are parallel to one anotherand are passed through a cleaning station 6a arranged in a housing 130.The auxiliary electrode 11b is connected to a voltage source 29.

In the discharge device shown in section in FIG. 9, the corona dischargeelement In corresponds in arrangement and design to that shown in FIG.8. The auxiliary electrodes 11 and 12, however, are stationary rows ofneedles. Furthermore, another auxiliary electrode 11 may be providedwithin the loop of the corona discharge element 1n. This auxiliaryelectrode 11' is a row of needles in the example shown in FIG. 9.

What is claimed is: I

1. A corona discharge device comprising an endless movable belt as atleast one corona discharge element, said belt having at least oneconductive layer at two discharge edges of the belt, a row of supportedelectrodes facing each discharge edge a uniform distance therefrom for aportion of said belt which with said portion form a discharge zone and acleaning device through which said belt moves positioned outside of thedischarge zone and extending along said belt.

2. The corona discharge device as claimed in claim 1, including a blockin which the corona discharge device is arranged, two angle sheetssecuring said rows of electrodes to this block, guide rolls for drivingsaid belt positioned in said block, which is arranged in a manner suchthat the corona discharging element is parallel to the web to becharged.

3. The corona discharge device as claimed in claim 2, in which saidblock defines two recesses between which is a bridge on which theendless belt is supported, each edge of the belt facing a row ofelectrodes resting on the side walls of the recesses and protruding fromthe side walls in the direction of the belt edges.

4. The corona discharge device as claimed in claim 3, in which saidblock defines one or more longitudinal channels in connection with saidrecesses via several bores to remove ozone produced by the dischargeprocess.

5. The corona discharge device as claimed in claim 2, in which the blockdefines a recess on its reverse side to the discharging zone, saidrecess housing said cleaning device and closed with a lid.

6. The corona discharge device as claimed in claim 5, in which saidcleaning device consists of brushes distributed over the entire distancebetween the guide rolls in the recess on the reverse side of the block.

7. The corona discharge device as claimed in claim 1, in which the belthas at least one endless corrugation in its length direction.

8. The corona discharge device as claimed in claim 7, in which thebridge defines a longitudinal recess in form and depth corresponding tothe corrugation of the belt, said longitudinal recess and corrugationbeing in engagement to secure said belt against lateral displacement.

9. The corona discharge device as claimed in claim 7, including a plateconnected at both ends of the block, said plate having a rib engagingthe corrugation of the belt to secure said belt against displacement l1, including a block in which the corona discharge device is arrangedand defining recesses for a cleaning device for each electrode belt onthe side of the block opposite to the discharging zone.

13. The corona discharge device as claimed in claim 11, including ablock in which the corona discharge device is arranged, said blockdefining two recesses between which is a bridge and outer side wallshaving horizontal ends, said electrode belts having corrugations whichare in engagement with corresponding nips at the inner faces of thehorizontal ends of the outer side walls of the block to prevent lateraldisplacement of the electrode belts.

14. The corona discharge device as claimed in claim 13, including meansfor driving said electrode belts in a direction opposite to thedirection of rotation of the discharge element.

15. The corona discharge device as claimed in claim 13, including meansfor driving said electrode belts in the same direction as the dischargeelement at different speeds.

16. The corona discharge device as claimed in claim 1, in which thecorona discharge element consists of several rigid individual elementsconnected with one another by joints.

17. The corona discharge device as claimed in claim 1, in which thecorona discharge element consists of several rigid individual elementsmounted on a flexible tape.

18. The corona discharge device as claimed in claim 1, in which thecorona discharge element has at least one further electroconductivelayer separated from the discharge edges of the corona discharge elementby an insulating layer and connectable to a controlled voltage sourcewith a variable output voltage.

19. The corona discharge device as claimed in claim 1, in which thecorona discharge element and the electrodes are toothed.

20. The corona discharge device as claimed in claim 1, in which theendless belt is arranged parallel to the rows of electrodes in the formof an endless belt.

21. The corona discharge device as claimed in claim 20, including ahousing containing cleaning devices for both belts, said housing partlyincluding one of the guide rolls about which the belts are turned.

22. The corona discharge device as claimed in claim 1, including a blockin which the corona discharge device is arranged, said block definingtwo recesses between which is a bridge and side walls and controlelectrodes arranged on both sides of each row of electrodes and in thebridge of the block.

23. The corona discharge device as claimed in claim 22, in which acontrol electrode is provided in the bridge of the block above thecorona discharge element and is arranged in the same plane with thecontrol electrodes on one side of the rows of electrodes.

24. The corona discharge device as claimed in claim 22, in which eachrecess of the block has a control electrode arranged on the bottom ofthe recess.

25. The corona discharge device as claimed in claim 22, in which theelectrodes of the corona discharging device are connected to severalvoltage supply parts with different output voltages.

1. A corona discharge device comprising an endless movable belt as atleast one corona discharge element, said belt having at least oneconductive layer at two discharge edges of the belt, a row of supportedelectrodes facing each discharge edge a uniform distance therefrom for aportion of said belt which with said portion form a discharge zone and acleaning device through which said belt moves positioned outside of thedischarge zone and extending along said belt.
 1. A corona dischargedevice comprising an endless movable belt as at least one coronadischarge element, said belt having at least one conductive layer at twodischarge edges of the belt, a row of supported electrodes facing eachdischarge edge a uniform distance therefrom for a portion of said beltwhich with said portion form a discharge zone and a cleaning devicethrough which said belt moves positioned outside of the discharge zoneand extending along said belt.
 2. The corona discharge device as claimedin claim 1, including a block in which the corona discharge device isarranged, two angle sheets securing said rows of electrodes to thisblock, guide rolls for driving said belt positioned in said block, whichis arranged in a manner such that the corona discharging element isparallel to the web to be charged.
 3. The corona discharge device asclaimed in claim 2, in which said block defines two recesses betweenwhich is a bridge on which the endless belt is supported, each edge oFthe belt facing a row of electrodes resting on the side walls of therecesses and protruding from the side walls in the direction of the beltedges.
 4. The corona discharge device as claimed in claim 3, in whichsaid block defines one or more longitudinal channels in connection withsaid recesses via several bores to remove ozone produced by thedischarge process.
 5. The corona discharge device as claimed in claim 2,in which the block defines a recess on its reverse side to thedischarging zone, said recess housing said cleaning device and closedwith a lid.
 6. The corona discharge device as claimed in claim 5, inwhich said cleaning device consists of brushes distributed over theentire distance between the guide rolls in the recess on the reverseside of the block.
 7. The corona discharge device as claimed in claim 1,in which the belt has at least one endless corrugation in its lengthdirection.
 8. The corona discharge device as claimed in claim 7, inwhich the bridge defines a longitudinal recess in form and depthcorresponding to the corrugation of the belt, said longitudinal recessand corrugation being in engagement to secure said belt against lateraldisplacement.
 9. The corona discharge device as claimed in claim 7,including a plate connected at both ends of the block, said plate havinga rib engaging the corrugation of the belt to secure said belt againstdisplacement transverse to the direction of its movement.
 10. The coronadischarge device as claimed in claim 1, in which the two rows ofelectrodes consist of densely arranged needles.
 11. The corona dischargedevice as claimed in claim 1, in which the two rows of electrodesconsist of endless circulating belts.
 12. The corona discharge device asclaimed in claim 11, including a block in which the corona dischargedevice is arranged and defining recesses for a cleaning device for eachelectrode belt on the side of the block opposite to the dischargingzone.
 13. The corona discharge device as claimed in claim 11, includinga block in which the corona discharge device is arranged, said blockdefining two recesses between which is a bridge and outer side wallshaving horizontal ends, said electrode belts having corrugations whichare in engagement with corresponding nips at the inner faces of thehorizontal ends of the outer side walls of the block to prevent lateraldisplacement of the electrode belts.
 14. The corona discharge device asclaimed in claim 13, including means for driving said electrode belts ina direction opposite to the direction of rotation of the dischargeelement.
 15. The corona discharge device as claimed in claim 13,including means for driving said electrode belts in the same directionas the discharge element at different speeds.
 16. The corona dischargedevice as claimed in claim 1, in which the corona discharge elementconsists of several rigid individual elements connected with one anotherby joints.
 17. The corona discharge device as claimed in claim 1, inwhich the corona discharge element consists of several rigid individualelements mounted on a flexible tape.
 18. The corona discharge device asclaimed in claim 1, in which the corona discharge element has at leastone further electroconductive layer separated from the discharge edgesof the corona discharge element by an insulating layer and connectableto a controlled voltage source with a variable output voltage.
 19. Thecorona discharge device as claimed in claim 1, in which the coronadischarge element and the electrodes are toothed.
 20. The coronadischarge device as claimed in claim 1, in which the endless belt isarranged parallel to the rows of electrodes in the form of an endlessbelt.
 21. The corona discharge device as claimed in claim 20, includinga housing containing cleaning devices for both belts, said housingpartly including one of the guide rolls about which the belts areturned.
 22. The corona discharge device as claimed in claim 1, includinga block in which the Corona discharge device is arranged, said blockdefining two recesses between which is a bridge and side walls andcontrol electrodes arranged on both sides of each row of electrodes andin the bridge of the block.
 23. The corona discharge device as claimedin claim 22, in which a control electrode is provided in the bridge ofthe block above the corona discharge element and is arranged in the sameplane with the control electrodes on one side of the rows of electrodes.24. The corona discharge device as claimed in claim 22, in which eachrecess of the block has a control electrode arranged on the bottom ofthe recess.